1. Field of the Invention
The invention relates to plastisols based on PMMA polymer masses plasticized by means of the addition of a plasticizer, which additionally also contain a cross-linking component.
2. Discussion of the Background
Plastisols are two-phase systems consisting of plastic or synthetic resin particles and a suitable plasticizer or plasticizer mixture. They can theoretically be produced from many different plastics, but the technically most important applications have always been restricted to polymer systems. Copolymerizates of polyvinyl chloride and other vinyl-unsaturated monomers are known and find broad application. A disadvantage of these materials is their chlorine content, which has a corrosive effect under certain conditions. These plastisols are also known as PVC plastics, plastisols, organosols, or plastigels (see also Ullmann""s Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A21, pages 734-737, VCH 1992; Becker-Braun, Kunststoff-Handbuch (Plastics Handbook), 2nd edition, vol. 2/2, pages 1077-1090, C. Hanser 1986; F. Mark et al., Encyclopedia of Polymer Science and Engineering, 2nd Ed. Supplem. Vol. pages 568-643, Wiley-Interscience 1989, and Saechtling, Kunststoff Taschenbuch (Plastics Pocket Book), Carl Hanser Verlag, Munich, 26th edition, (1995), pages 406 ff.).
Copolymerizates based on methyl methacrylate and butyl methacrylate, so-called PAMA plastisols, are used with great success for plastisols in the automotive sector. The plastisols are used as adhesive plastisols, sealing masses, welding pastes, and undercoats, and also as floor coverings. But plastisols on an acrylate basis have also been available for some time (cf. DE-PS 934 498, FR-A 2,291,248). The latter state of the art is based on the recognition that coordination of the glass temperature Tg, particle size, and composition of the polymer particles, on the one hand, and of special plasticizers on the other hand, is required for the production of acceptable PAMA plastisols.
In this connection, the rule is established that the average grain size of the polymerizate must be all the greater, the lower its glass temperature Tg. The average grain size of the polymer used must be between 0.1 xcexcm and 500 xcexcm, preferably between 0.3 and 20 xcexcm, according to the FR-A. Both suspension-polymerized and emulsion-polymerized acrylic polymers are indicated as suitable primary particles.
DE-A 25 43 542 also describes PAMA plastisols, preferably with basic comonomers in a particle size range of 0.1 to 200 xcexcm, which are preferably obtained as emulsion polymerizates. Particles from 0.1 to 20 xcexcm, particularly up to 10 xcexcm, are especially preferred, but can only be produced by emulsion polymerization, according to this reference. Another modification of the PAMA plastisols is proposed in DE-A 27 72 752 and 29 49 954. This involves core/shell polymerizates with a plasticizer-compatible core and a shell which is less compatible with plasticizer. In U.S. Pat. No. 4,558,084, a plastisol based on a copolymerizate of methyl methacrylate and itaconic acid, i.e. itaconic acid anhydride is described; it is said to have particularly good adhesion on electrophoretically pre-treated metal surfaces. Recently, floor coverings based on PAMA plastisols have been proposed, in which a pure polymethyl methacrylate (PMMA) is used as the matrix material, used partly in the form of emulsion polymerizate, partly in the form of suspension polymerizate (DE-A 39 03 669).
As was shown in EP-A 477 708, a plastisol with improved product quality and greater application breadth can be obtained by simultaneous use of spray-dried polymer dispersions and bead polymerizate obtained in suspension polymerization, at least one of which represents a methyl methacrylate copolymerizate.
Further studies in the field of PAMA plastisols served to improve the adhesion strength, particularly on metallic substrata, for example by also using 2.5-10% by weight polyamide resins (polyaminoamides) (cf. EP-A 533 026). EP-A 121.759 also recommends polyamide resins as protective colloids for improving the storage stability of plastisols. Other references aim at improving the adhesion properties, for example by installation of glycidyl (meth)acrylate, among other things, into the PAMA polymer (US-PS 5,120,795), or at improving the plasticizer compatibility by incorporation of isobutyl methacrylate as a comonomer (EP-A 477 708; EP-A 539.031). DE-OS 24.54.235 (Teroson GmbH) describes non-cross-linked plastisols of methyl methacrylate and butyl methacrylate for use as undercoats in automobile construction. DE-OS 25.43.542 (Rxc3x6hm GmbH) describes a non-cross-linked copolymer of methyl methacrylate and butyl methacrylate as well as vinyl imidazole as the adhesion mediator. Chalk is used as the filler, the plastisol is used for coating metals. DE-OS 28.12.016 (du Pont) describes non-aqueous, non-cross-linked plastisols of methyl methacrylate and methacrylic acid. After they gel, a clear film is obtained. DE-OS 27.22.752 (Rxc3x6hm GmbH) describes a plastisol with a core/shell structure. The core consists of a copolymer of methyl methacrylate, butyl acrylate, and butyl methacrylate, the shell consists predominantly of methyl methacrylate. DE-OS 28.812.014 (du Pont) describes a photosensitive plastisol of methyl methacrylate and methacrylic acid.
DE-OS 29.101.53 (Bayer AG) describes a cross-linked acrylate rubber, which is worked into a dispersion. DE-OS 39.00.933 (BASF AG) describes plastisols of methyl methacrylate, alkyl methacrylate, unsaturated esters of monocarboxylic or dicarboxylic acids, glycidyl methacrylate, and monomers which contain basic nitrogen groups or hydroxy groups. DE-OS 40.300.80 describes a mixture of a spray-dried dispersion and a spray-dried suspension of methyl methacrylate and alkyl methacrylates, on the one hand, and butyl methacrylate and methyl methacrylate, on the other hand. An adhesion-imparting monomer, for example vinyl imidazole, can be optionally added. EP 544 201 (Nippon Zeon Co. Ltd.) describes a plastisol mixture of alkyl methacrylates and dienes, such as butadiene and isoprene. Sulfur or ZnO serves as an optional cross-linking agent. Cross-linking takes place via the carboxyl groups.
From DE 39 03 670 (Pegulan AG), floor coverings are known which consist of PMMA and benzyloctylphthalate as the plasticizer. These plastisol mixtures are heated to 120xc2x0 C. and immediately processed further to produce the floor covering. There is no information concerning stabilization of the plastisol.
It is disadvantageous in this connection that a large number of technically important and frequently used plasticizers cannot be used for PMMA plastisols, since the plastisols produced with them are not sufficiently stable. The relevant parameter here is the viscosity increase of the plastisols, which makes processing of the products more difficult.
One object of the present invention is to provide chlorine-free plastisols with good mechanical properties, a broad freedom of movement for processing, a constant viscosity over the processing period, and one in which the added plasticizer does not sweat out. Sweating results in unattractive color changes at the surface of the plastisols and impairs their usage properties.